Spermatogenesis in Udonella (Platyhelminthes, Udonellidea) and the phylogenetic position of the genus.

Spermatids are joined (cytophores). Two centrioles lateral and/or proximal to the nucleus form axonemes, arranged parallel with each other, that extend into a short distal cytoplasmic process. The nucleus elongates, and mitochondria fuse to form a single elongate mitochondrion. The part of the spermatid containing the axonemes, nucleus and mitochondrion elongates to a thread-like spermatozoon, surrounded at its base (at the level near the basal bodies) by an arching membrane where the sperm detaches from the residual cytoplasm. At no stage are there free flagella (except for short free flagellar tips), an intercentriolar body or a median cytoplasmic process. A few peripheral microtubules and a compact dense body, not membrane bound and closely associated with the mitochondrion, were seen in spermatids. Spermiogenesis in Udonella is interpreted as derived from proximo-distal fusion by a shifting of the basal bodies deeper into the cytoplasm and a consequent loss of the median cytoplasmic process. Both the ultrastructure of the protonephridia and spermiogenesis indicate that Udonella does not represent the sister group of all other neodermatans, but is a secondarily modified "advanced" neodermatan. Lack of convincing synapomorphies indicates that the taxon Cercomeridea as a defined by Brooks (1989a, b) is invalid.